Multi-cavity tubes for air-over evaporative heat exchanger

ABSTRACT

An air-over evaporative heat exchanger with multi-lobed or “peanut” shaped tubes replacing conventional round or elliptical tubes. The tubes have a narrow horizontal cross section and tall vertical cross section to allow the multiplication of surface area in the same coil volume while maintaining or increasing the open-air passage area. This configuration allows the coil to have an overall external heat transfer coefficient much higher than a conventional coil, while the tube shape allows the use of thinner material, reducing the weight and cost of the heat exchanger.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to evaporative air-over heat exchangers.

Description of the Background

It is well known that elliptical tubes work well for evaporative heatexchangers. Increasing the heat exchanger tube density works well forsystems that have no airflow over the coil, while increasing theexternal surface area using extended fins works well in systems thathave airflow over the coil. However, both of these methods increase theweight of the heat exchanger coil and consequent cost per heat exchangercompared to conventional tube-coil designs since the tubes are requiredto have a minimum wall thickness to operate under internal pressurewithout deforming.

SUMMARY OF THE INVENTION

This invention serves to solve the problem of increased weight and costwith incremental improvements in capacity by improving the thermalcapacity while decreasing the cost for equivalent thermal capacity witha special tube shape and pattern that increases the prime surface areain contact with the airstream thereby improving thermal capacity, at thesame time decreasing the thickness of the heat exchanger tubes therebydecreasing the cost for equivalent thermal capacity. The effectivediameter of the tube is reduced by the design of the invention, whichallows the tube wall to be reduced in thickness for the same internalpressure. The open air face area to tube face area ratio determines to alarge extent the effectiveness of the heat exchanger. If this ratio istoo low, the heat exchanger will have an undesirable airside pressuredrop, lowering its effectiveness in an evaporative heat exchanger. Thiseffect is more pronounced in evaporative heat exchangers than in a dryair heat exchanger because of the water-air interaction. The tube shapeand pattern of the invention serves to keep this ratio equal to or lowerthan conventional heat exchangers of the same volume (i.e., coil volume,that is, the volume defined by the outer dimensions of the coil, L×W×H)while increasing the surface area of the coils. The combination ofincreasing the coil surface area, reducing the tube wall thickness, andmaintaining or decreasing the airside pressure drop using the new tubedesign of the invention serve to create a heat exchanger with superiorthermal efficiency and cost effectiveness.

Therefore, there is provided according to various embodiments of theinvention multi-lobed tubes that may be used in place of single round orelliptical-shaped tubes of prior art heat exchangers. These multi-lobedtubes are tall and narrow in vertical cross section. The multi-lobedtubes may have 2, 3, 4 or more lobes per tube. The multi-lobed shapeallows the tubes to have a smaller air-face profile and thinner wallwhile maintaining the working pressure limit and outside surface areaper tube. The narrow air-face profile also allows many more tubes toexist in the same heat exchanger volume while maintaining or decreasingthe open air face area to tube face area ratio to maintain or decreasethe airside pressure drop and maintain or increase the airflow volumeper horsepower. Heat exchangers having the tube design of the presentinvention will work equally well as fluid coolers or refrigerantcondensers.

Accordingly, there is presented according to an embodiment of theinvention an air-over evaporative heat exchanger coil having multi-lobedtubes that have the same or higher surface area as a heat exchanger coilof the same size/volume with conventional round or elliptical tubes.

Accordingly, there is presented according to an embodiment of theinvention an air-over evaporative heat exchanger coil having multi-lobedtubes that use much thinner tube walls than a conventional single tubeof the same outside surface area.

Accordingly, there is presented according to an embodiment of theinvention an air-over evaporative heat exchanger coil having an open airface area to tube face area ratio equivalent or greater than aconventional heat exchanger coil of the same size/volume withconventional round or elliptical tubes.

Accordingly, there is presented according to an embodiment of theinvention an air-over evaporative heat exchanger coil having tubesurface area significantly larger than a conventional heat exchangercoil of the same size/volume with conventional round or ellipticaltubes.

Accordingly, there is presented according to an embodiment of theinvention an air-over evaporative heat exchanger coil comprised of: aplurality of multi-lobed tubes arranged in a tube bundle.

There is further presented according to an embodiment of the inventionan air-over evaporative heat exchanger coil with multi-lobed tube havingexactly two lobes.

There is further presented according to an embodiment of the inventionan air-over evaporative heat exchanger coil with multi-lobed tubeshaving exactly three lobes.

There is further presented according to an embodiment of the inventionan air-over evaporative heat exchanger coil with multi-lobed tubes with100%-300% of the tube surface area of a coil having the same externaldimensions with 0.85 inch elliptical tubes.

There is further presented according to an embodiment of the inventionan air-over evaporative heat exchanger coil with multi-lobed tubes with25%-150% of the open-air passage area of a coil having the same externaldimensions with 0.85 inch elliptical tubes.

There is further presented according to an embodiment of the inventionan air-over evaporative heat exchanger coil with multi-lobed tubeswherein the major axis of the tube is tilted 0 to 25 degrees relative tovertical.

There is further presented according to an embodiment of the inventionan evaporative heat exchanger for cooling or condensing a process fluid,comprising: an indirect heat exchange section; a water distributionsystem located above the indirect heat exchange section and configuredto spray water over the indirect heat exchange section; wherein theindirect heat exchange section comprises a process fluid inlet headerand a process fluid outlet header, and an array of tubes multi-lobedtubes connecting said inlet header and said outlet header, said tubesfurther having lengths extending along a longitudinal axis; theevaporative heat exchanger also including a plenum where waterdistributed by said water distribution system and having received heatfrom said indirect section is cooled by direct contact with air movingthrough said plenum; a water recirculation system, including pump andpipes, configured to take water collecting at the bottom of said plenumand deliver it to said water distribution system; and an air moverconfigured to move ambient air into said plenum and up through saidindirect section.

There is further presented according to an embodiment of the invention,a heat exchange tube bundle in which the multi-lobed tubes are straightand are each connected at a first end to a process fluid inlet headerand at a second end to a process fluid outlet header.

There is further presented according to an embodiment of the invention aheat exchange tube bundle in which the multi-lobed tubes are serpentineand each serpentine tube comprises a plurality of lengths connected ateach end to adjacent lengths of the same serpentine tube by tube bendsand connected at one end of a serpentine tube to a process fluid inletheader, and at a second end to a process fluid outlet header.

BRIEF DESCRIPTION OF THE DRAWINGS

The subsequent description of the preferred embodiments of the presentinvention refers to the attached drawings, wherein:

FIG. 1 is a cutaway side view of a prior art evaporative heat exchanger.

FIG. 2 is a cutaway perspective view of a prior art evaporative heatexchanger.

FIG. 3 shows an outside perspective view of a conventional prior artelliptical evaporative heat exchanger tube.

FIG. 4 shows a cross-sectional view of the conventional prior artelliptical evaporative heat exchanger tube of FIG. 3.

FIG. 5 is a representation of a cross-sectional view of a conventionalprior art evaporative heat exchanger tube bundle having ellipticaltubes.

FIG. 6 is another representation of a cross-sectional view of aconventional prior art evaporative heat exchanger tube bundle havingelliptical tubes.

FIG. 7 is a graphical representation of the open air face area to tubeface area for a conventional prior art evaporative heat exchanger tubebundle having elliptical tubes.

FIG. 8 shows a cross-sectional view of a 2-lobed or “peanut”-shaped heatexchange tube according to an embodiment of the invention.

FIG. 9 shows an outside perspective view of a 2-lobed or “peanut”-shapedheat exchange tube according to an embodiment of the invention.

FIG. 10 is a representation of a cross-sectional view of an evaporativeheat exchanger tube bundle having 2-lobed or “peanut”-shaped heatexchange tubes according to an embodiment of the invention.

FIG. 11a is another representation of a cross-sectional view of anevaporative heat exchanger tube bundle having 2-lobed or “peanut”-shapedheat exchange tubes according to an embodiment of the invention.

FIG. 11b is another representation of a cross-sectional view of anevaporative heat exchanger tube bundle having 2-lobed or “peanut”-shapedheat exchange tubes according to an embodiment of the invention.

FIG. 12 shows a graphical representation of the open air face area totube face area for an evaporative heat exchanger tube bundle having2-lobed or “peanut”-shaped heat exchange tubes according to anembodiment of the invention.

FIG. 13 shows several multi-tube heat exchange tube unit and“peanut”-type tube configurations according to further alternateembodiments of the invention.

FIG. 14 shows the effect of densifying a coil by using narrower tubes ofthe same diameter and thickness.

FIG. 15 shows the relationship between tube width and required steeltube thickness for equivalent working pressure for round and “squashed”1.05″ diameter tubes versus “peanut” shaped tubes with 25% more externalsurface area.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show an induced draft single cell evaporative cooleraccording to the prior art. Fan 101 draws air into the unit and forcesit out the top of the unit. Below the fan is a water distribution system103 that distributes water over the tube coil 105. The tube coil is madeof an array of serpentine elliptical tubes 107. Each length of tube 109is connected at its ends to an adjacent higher and/or lower tube lengthby a tube bend 111. Process fluid to be cooled enters the tubes via aninlet header 113 and exits the tubes via an outlet header 115. Beneaththe tube coil is the plenum 117, where air enters the unit and the waterthat is delivered to the unit via the water distribution system 103 iscooled via direct heat exchange with the air, collects at the bottom andrecirculated to the top via water recirculation system 119.

FIGS. 3 and 4 shows a conventional evaporative heat exchanger ellipticaltube 107 of the type used in the prior art heat exchanger of FIGS. 1 and2. A working fluid such as water, glycol, or ammonia 15 is containedwithin the tube wall 16. Water droplet-filled air 17 flows around thetube from bottom to top. FIGS. 5 and 6 show how a plurality of tubes ofthe type shown in FIGS. 3 and 4 are typically arranged in a tube bundlein a heat exchanger of FIGS. 1 and 2. Multiple tubes 18 a,b, etc., aregenerally arranged in a patterned allow water droplet-filled air 19 topass around the tubes under the force of gravity. The ratio of open airface area 20 to tube face area for this arrangement is shown in FIG. 7,according to standard tube sizing and spacing shown in FIG. 6. Tubes ofthis type are typically formed from round 1.05 inch diameter tubinghaving a tube wall thickness of 0.055 inches, which are thenmechanically “squeezed” into an ellipse having a minor diameter of 0.850inches. FIG. 7 shows graphical representation of the open air face area20 to tube face area 21 for a standard evaporative heat exchanger tubebundle with elliptical tubes having a tube width of 0.850 inches.

FIGS. 8 and 9 show two-lobed “peanut”-shaped tubes according to anembodiment of the invention. As with prior art tubes, working fluid suchas water, glycol, or ammonia 1 is contained within the tube wall 2.Water droplet-filled air 3 flows around the tube from bottom to top.According to a preferred embodiment, the tube height is 1.790 inches,the tube width at the widest cross-section of each lobe is 0.375 inches.However, these dimensions should not be deemed to limit the invention,as multi-lobed tubes of any dimensions may be used according to theinvention, including tube heights of 1.250 to 2.500 inches with lobecross sections of 0.200 to 0.500 inches. The cross-sectional shape ofthe lobes may be range from teardrop to nearly circular to circular.According to a preferred embodiment opposing inside surfaces of thetubes are welded together at the pinch, i.e., where the inside tubesurfaces meet (roughly at the center of the tube in the case oftwo-lobed tubes). According to various embodiments, the tubes may befinless or finned. Tube wall width is preferably 0.055 inches, but canrange from 0.005 inches to 0.06 or greater. In any event, embodiments ofthe invention can withstand working pressures of 300 psi to 400 psi andbeyond.

FIGS. 10, 11 a and 11 b show cross-sectional views of evaporative heatexchanger tube bundles including an arrangement of 2-lobed or“peanut”-shaped tubes of FIGS. 8 and 9. According to this embodiment,the tube bundle has twice the prime external tube surface area of aconventional heat exchanger tube bundle (1.05 inch round tubes or 0.85elliptical tubes) of the same volume (i.e., coil volume, that is, thevolume defined by the outer dimensions of the coil, L×W×H). Multipletubes 4 a, 4 b, etc., are arranged according to the pattern shown toallow water droplet-filled air 5 to pass around the tubes. According toa preferred embodiment, spacing between vertically adjacent rows oftubes (measured center to center) is 102%-106% of the tube height, morepreferably 104% of the tube height. Preferred spacing betweenhorizontally adjacent tubes (measured center to center) is 305% to 320%of the lobe width, more preferably 310% to 312% and most preferably311%.

FIG. 12 shows graphical representation of the open air face area 6 totube face area 7 for a “peanut” unit evaporative heat exchanger tubebundle of the present invention. The open air face area is nearly thesame as for a prior art heat exchange coil of the same volume so thatthe same amount of air can flow through the coil without changing thefan size or power. However, a coil according to the present inventionwith two-lobed or “peanut” shaped tubes has twice the prime externaltube surface area of a conventional evaporative heat exchanger tubebundle of the same volume.

FIG. 13 shows additional multi-lobe tube embodiments. According tovarious embodiments, the lobed-tubes may have 2, 3, 4 or more lobes. Andthe longitudinal axis of the tube cross-section may be tilted from 0 to25 degrees from vertical.

FIG. 14 shows the effect of densifying a coil by using progressivelynarrower or “squashed” tubes of the same diameter and thickness, i.e.,starting with round tubes of 1.05 inch diameter (farthest-right pointson the chart), the total coil surface area, the cost, the thermalcapacity and the number of tubes was examined for a tube coil having thesame volume/outside dimensions. The bottom axis reflects decreasing tubewidth, from right to left, as 1.05 inch tubes having tube wall thicknessof 0.055 inches are squashed into increasingly elliptical tubes. Theleft axis shows the percentage coil surface, cost, thermal capacity ornumber of tubes, relative to a coil containing standard elliptical tubeshaving a width of 0.85 inches. This chart shows that Cost is directlyproportional to the thermal capacity. What is not reflected in thischart is that the working pressure limit of the coils decreasesdramatically as the tube is squashed more and more, see FIG. 15.

FIG. 15 shows the relationship between tube unit profile width andrequired steel tube thickness for equivalent working pressure for roundand “squashed” 1.05″ diameter tubes versus “peanut” shaped tubes with25% more external surface area. The bottom axis shows tube width,starting on the far right 1.2 inches. The left axis shows the requiredtube wall thickness for safe operation at 300 psi working pressure. Theline that extends from the bottom right quadrant of the chart to the topleft shows how the tube thickness required for operation at 300 psi goesfrom approximately 0.015 inches for a round 1.05 inch tube, toapproximately 0.055 inches for an elliptical tube squashed from 1.05inches to 0.85 inches, to approximately 0.080 inches for an ellipticaltube squashed from 1.05 inches to 0.25 inches. In short, this line showsthat as a 1.05 inch tube is squashed (in order for example to fit moretubes in a coil), the thickness of the tube wall necessary to maintainworking pressure of 300 psi increases dramatically, thus increasingweight, and material and manufacturing costs. However, FIG. 15 alsoshows, surprisingly, that two and three-lobed peanut shaped tubes of thepresent invention have unexpectedly and significantly lower tube wallthickness requirements in order to operate at 300 psi working pressure.For example, a two-lobed tube having a height of 1.72 inches requires atube wall thickness of only 0.048 inches, which is less than the 0.055tube wall thickness of prior art 0.85 elliptical tubes. A two-lobed tubehaving a height of 1.51 inches requires a tube wall thickness of only0.036 inches for safe operation at 300 psi working pressure, and athree-lobed tube 1.72 inches in height requires a tube wall thickness ofonly 0.005 inches to operate safely at 300 psi working pressure.

1. An air-over evaporative heat exchanger coil comprised of: a plurality of multi-lobed tubes arranged in a tube bundle.
 2. The device according to claim 1 wherein the multi-lobed tubes have exactly two lobes.
 3. The device according to claim 1 wherein the multi-lobed serpentine tubes have exactly three lobes.
 4. The device according to claim 1 with 100%-300% of the tube surface area of a coil having the same external dimensions with 0.85 inch elliptical tubes.
 5. The device according to claim 1 with 25%-150% of the open-air passage area of a coil having the same external dimensions with 0.85 inch elliptical tubes.
 6. The device according to claim 1 wherein the major axis of the tube is tilted 0 to 25 degrees relative to vertical.
 7. The device according to claim 2 wherein the major axis of the tube is tilted 0 to 25 degrees relative to vertical.
 8. The device according to claim 3 wherein the major axis of the tube is tilted 0 to 25 degrees relative to vertical. 9.-13. (canceled)
 14. The device according to claim 1, wherein said tubes are finned.
 15. (canceled)
 16. The device according to claim 1, said tubes having tube heights of 1.250 to 2.500 inches with lobe widths of 0.200 to 0.500 inches and tube wall width from 0.005 inches to 0.055 and wherein said tubes can withstand working pressure of 300 psi.
 17. (canceled)
 18. The device according to claim 1, said tubes having tube heights of 1.790 inches, a tube width at a widest cross-section of each lobe of 0.375 inches, and a tube wall width of 0.055 inches, and wherein said tubes can withstand working pressures of 300 psi.
 19. (canceled)
 20. The device according to claim 1 wherein the multi-lobed tubes are straight and each length of straight multi-lobed tube is connected at a first end to a process fluid inlet header and at a second end to a process fluid outlet header.
 21. The device according to claim 1 wherein the multi-lobed tubes are serpentine and each serpentine tube comprises a plurality of lengths connected at each end to adjacent lengths of the same serpentine tube by tube bends and wherein a first end of each said serpentine tube is connected at one end to a process fluid inlet header, and at a second end to a process fluid outlet header. 